Process for coating stainless steel



United States Patent 3,210,220 PROCESS FOR COATING STAINLESS STEELNorman E. Clegg, 12522 Foster Road, Los Alamitos, Calif., and William J.Greening, 6815 Monlaco Road, Long Beach, Calif. No Drawing. Filed July30, 1962, Ser. No. 214,450 5 Claims. (Cl. 148-621) I This inventionrelates to metal coating and, more particularly, to a method adapted forthe coating of stainless steel.

The process of the invention is particularly adapted to the coating ofstainless steel to provide a tightly adhering oxide film. The process islimited in its application to stainless steel alloys of iron andchromium. The process is not adaptable to the coating of iron or ironalloys containing no significant amount of chromium. Iron alloys knownas stainless steel normally contain from to 30% chromium and may containfrom 1 to 30% nickel. The process of the invention may be used to coatchromium steel alloys having less than 10% chromium, for example,chromium steel alloys containing 6% chromium; but, for the most part,the process will be found to have its greatest advantages in the coatingof alloys of higher chromium content. The presence of smaller amounts ofother alloy materials, such as molybdenum, copper, aluminum, cobalt,titanium, manganese and the like, is permissible, provided a significantamount of chromium is contained in the steel alloy composition.

The process of the invention comprises subjecting a steel-chromium alloysurface of an object to the action of a concentrated sulfuric or nitricacid water solution of chromium trioxide, CrO for a length of timenecessary to form thereon the desired thin oxide coating. The solutionis held at an elevated temperature and must be highly acid with thesulfuric or nitric acid making up at least of the aqueous acid solutionon a weight basis. It has been found that if less than 20% acidsolutions are employed, the metal surface being treated will be corrodedby the acid bath. One of the advantages of the process is that a colorcoating can be produced while maintaining a mirror-like finish on thesurface of the stainless steel base metal. Also, solutions containingless than about 20% acid fail to provide the required concentration oftetrachromic acid which latter acid is thought to be responsible for thefilm formation. Preferably, the mineral acid is used in the process bathin an amount of to 70% on a Weight basis. Mineral acid solutions inexcess of 70% are to be avoided, though operative, as such solutionswill cause the rejection of some of the chromium trioxide from thesolution. Sulfuric acid is preferred to nitric acid and is generallyused. Hydrochloric acid is not suitable for use in the process becausethe chloride ion is very detrimental to stainless steel.

The temperature of the acid bath for the formation of the film should bein excess of 150 F. and is usually within the range of 175 to 210 F. Atemperature within the more limited range of 190 to 200 F. is generallyoptimum, but this will vary with the composition of the bath to someextent. The relative low temperature of the acid bath protects thesurface finish of the stainless steel and avoids alteration of thephysical characteristics of the base metal.

The immersed alloy surface is left in the sulfuric acid solution to formthe oxide coating, being removed when the coating has acquired thedesired color. It has been found that almost an unlimited number ofshades of blacks, blues, golds, violets, greens and yellows (in thatorder) can be procured by varying the time of immersion and thetemperature of the bath. The color change is fairly rapid and normallythe alloy surface need be imice mersed only a few minutes, for example,two to six minutes. The particular length of time required to obtain acoating of a desired color is dependent upon bath composition andtemperature, but seldom needs to exceed ten or fifteen minutes.

It has been found that a small amount of ammonium molybdate (NH MoO isdesirably included in the acid bath to improve the luster of thefinished product. Preferably, the ammonium molybdate is used in anamount from a trace to an optimum of 6.5 to 8.5 grams per liter of thesulfuric acid bath.

The effectiveness of the processing bath requires that the bath in itspreparation be held at an elevated temperature for a fairly long periodof time before use. The purpose of the prolonged heating is to permitwhat is believed to be the formation of tetrachromic acid. Theprocessing bath after a period of time at an elevated temperature takeson a dark reddish-black color which is though to be an indication of thepresence of the tetrachromic ion Cr O= As an example, in the preparationof a typical bath, 500 ml. of 9 normal H 50 is added slowly to 500 ml.of water. 7.5 grams of ammonium molybdate is added.

The solution is heated to 190F. and 850 grams of chromium trioxide (CrOadded, this being in excess of the amount that will dissolve. Amounts ofchromium trioxide less than that which will dissolve may be used butwith the result that proportionally less of the tetrachromic acid willbe produced. Desirably, at least 775 grams of chromium oxide is added toa liter of the acid solution. 775 to 850 grams of chromium trioxide is adesired range. An amount of about 275 grams is at the lower limit andwill require an exorbitant length of time to obtain results. The bath isthen held at the elevated temperature for at least six hours, andgenerally for eight to twenty-four hours with increased advantages beingobtained up to twenty-four hours. The bath is then decanted to separatethe chromium trioxide crystals that did not go into the solution. In theprocessing bath preparation, the prolonged heating is carried on at atemperature of at least F., and preferably at a temperature within therange of 200 F. Temperature in excess of 200 F. may be used, forexample, as high as 250 F., but generally to little advantage. Theprolonged heating favors the formation of the tetrachromic acid.

The presence of the tetrachromic acid in the process bath is believed tobe responsible for the production of the thin oxide coating, thethickness of which is dependent upon time of immersion, composition andtemperature of the bath. The characteristic colors of the varying filmthicknesses are believed to be produced by an interference effectbetween two trains of light waves reflected by opposite surfaces of thethin oxide film.

The process of the invention provides a highly desirable passivation of:the stainless steel surface in the forming of a continuous, tightlyadhering, oxide film. The oxide film resulting from the process of theinvention affords a greater corrosion protection to the basic metal thanis possible with, for example nitric acid passivation. Because of thecolor development of the oxide film, which color is indicative of filmthickness, passivation is obvious to visual inspection. In nitric acidtreatment, there being no visual indication of the degree ofpassivation, passivation is checked by a separate test. 65

The oxide film possesses desirable antigalling prop erties. There is adecided tendency for contacting stainless steel parts to gall or sieve.The oxide coating being nonmetallic, actually serves as a lubricant toprevent selfwelding from occurring during times of high bearing loading,either in static or dynamic situations.

In addition to its own inherent lubricating properties,

the oxide coating will also serve as a lake or holding area for oils ordry lubricants such as graphite. Where used as a holding area forlubricants, the oxide coating desirably has a thickness greater thanZA/n, where A is the wavelength of the light produced by a sodium flameand n is the index of refraction of the oxide formed. The alloy contentof the stainless steel has some effect on the reffaction index.

The oxide film formed by the process is essentially a nonconductor ofelectricity. This property of the oxide film permits its use in variouscontrolled thicknesses as an insulator, semiconductors, capacitors,diodes, resistors and other electronic uses.

As pointed out above, it is possible with the process of the inventionto obtain various colors in the coating by controlling the coatingthickness. This-may be done visually, removing the object from theprocessing bath when the desired color is reached. Colors of varyingdegrees of black, blue, violet, gold, green and yellow are obtained inaccordance with the formula At'y/n with the light normal to the surfaceof the coating. Here '7 is the wavelength of the color desired and n is.the index of refraction.

It has been found that oxide coating of the invention enhances thewettability of glass to stainless steel, resulting in a superior bondingof the glass to the metal. It is believed that the oxide coating of theinvention provides a well or lake for the glass to bond to the stainlesssteel. The coating may also be applied to stainless steel as apretreatment to improve the adherence of paints and plastics such asTeflon.

The u/e ratio (where or is the solar 'absorptivity and e is theemittance) may be selected to give the desired reflectivity orabsorptivity characteristics to the coated stainless steel. -In one suchapplication, a Venetian blind type louver is made of the coatedstainless steel with the thickness of the oxide coating being selectedto accept or reject heat as desired. A very fiat reflectance curve canbe obtained through the ultraviolet, visible and infrared light rangesthrough use of the thin film properties of the oxide coating.

The coating may be applied .to stainless steel in selective areas orapplied to entire surface and selectively removed. The surface of asample on which it is desired to place a design is coated with an acidresistant lacquer or photographic resist after the part has been coatedto obtain the desired color. The piece with the pattern thus establishedis immersed in an electrolyte such as a 5% solution of sulfuric vacidand an electric alternating or direct current is applied to a leadelectrode and the piece. This procedure removes the coating in all areasthat are not masked with the resist and thus produces the design.

It is possible with the process of the invention to produce multicoloredsamples. The following is a descrip tion of the technique used. Thesurface is, for example, first exposed to a processing bath forapproximately two minutes to impart to the surface a black coating. Theoptimum immersion time to obtain a black coating will depend upon thecomposition of the bath and its temperature. Following the applicationof the black coating, the piece is rinsed in hot water and dried withhot air. The piece is then coated with a photographic resist and exposedto a negative of the desired pattern. The image of the pattern is thendeveloped in accordance with conventional techniques. The piece is thenimmersed in the bath for an additional two minutes or so and the exposedarea which was formerly black now changes to blue. The photographicresist is removed by an appropriate solvent such as xylene and the pieceonce again rinsed in hot water and dried with hot air. Another patternis applied to the work piece as before by first coating the work piecewith photographic resist. The resist is then exposed to the negative ofa desired pattern and thereafter the image developed. The piece with theimage thereon is then immersed for an additional two minutes (making asix minute total) and the area of the new pattern takes on a gold color.The piece Will now have patterns aligned in black, blue and gold. Toproduce additional colors, this technique is repeated over and over. Thenext color to be produced will normally be violet followed by green.

Although exemplary embodiments of the invention have been disclosedherein for purposes of illustration, it will be understood that variouschanges, modifications, and substitutions may be incorporated in suchembodiments without departing from the spirit of the invention asdefined by the claims which follow.

We claim:

1. A method of forming a coating on a steel-chromium allow surface of anobject, said method comprising: subjecting the alloy surface of theobject to the action of a highly acid solution containing tetrachromicion and the anion of sulfuric acid, said solution being at a temperaturewithin the range of to 210 F.; and leaving the alloy surface in thesulfuric acid solution until a desired color is produced at which timethe object is removed from the sulfuric acid solution.

2. A method in accordance with claim 1 wherein the acid solution isprepared by providing chromium trioxide in the amount of at least 275grams per liter of the sulfuric acid solution and heating the solutionfor at least eight hours at a temperature in excess of 180 F.

3. A method of forming a coating on a steel-chromium alloy surface of anobject, said method comprising: sub jecting the alloy surface to theaction of a treating solution and forming thereon a desired thincoating, said solu tion having been prepared by heating at least for sixhours a concentrated mineral acid aqueous solution of chromium trioxideat a temperature in excess of 180 F., said mineral acid being selectedfrom the group consisting of sulfuric acid and nitric acid with themineral acid making up at least 20 percent of the aqueous solution on aweight basis.

4. A method in accordance with claim 3 wherein the heating of thesolution used for the treatment of the chromium alloy surface is carriedon at a temperature within the range of -200 F. for a period of eight totwenty-four hours.

5. A method in accordance with claim 3 wherein the mineral acid ispresent in the aqueous solution in the amount of 25-70 percent on aweight basis.

References Cited by the Examiner UNITED STATES PATENTS 2,283,170 5/42Batcheller 148-621 2,312,066 2/43 Batcheller 148-62 2,773,623 12/56Schuster et al 148-62 2,858,244 10/58 Long 148-62 2,902,394 9/59Jeremias 148-621 2,991,205 7/61 Lincoln 148-62 FOREIGN PATENTS 275,7818/27 Great Britain. 483,551 4/38 Great Britain.

RICHARD D. NEVIUS, Primary Examiner.

WILLIAM D. MARTIN, Examiner.

1. A METHOD OF FORMING A COATING ON A STEEL-CHROMIUM ALLOW SURFACE OF ANOBJECT, SAID METHOD COMPRISING: SUBJECTING THE ALLOY SURFACE OF THEOBJECT TO THE ACTION OF A HIGHLY ACID SOLUTION CONTAINING TETRACHROMICION AND THE ANION OF SULFURIC ACID, SAID SOLUTION BEING AT A TEMPERATUREWITHIN THE RANGE OF 175 TO 210*F.; AND LEAVING THE ALLOY SURFACE IN THESULFURIC ACID SOLUTION UNTIL A DESIRED COLOR IS PRODUCED AT WHICH TIMETHE OBJECT IS REMOVED FROM THE SULFURIC ACID SOLUTION.